Thursday, April 26, 2012

Interpreting (A-a)PO2 in patients with COPD

The (A-a)PO2 difference is elevated in patients with hypoxemia due to shunt, diffusion limitation, or V/Q mismatch.  It is normal in patients with pure hypoventilation resulting in lower alveolar and lower arterial PO2 but no change in (A-a)PO2.  However, the (A-a)PO2 difference is difficult to interpret in patients with elevated PCO2 and V/Q mismatch; e.g., COPD. 
Why is this so?
Basically, this is due to the shape of the O2 dissociation curve.



















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